The Sun radiates energy around the visible and ultra-violet region. The Earth and its atmosphere absorb part of this energy, and part is reflected back into space. The part that gets absorbed helps to heat the Earth, and the Earth in turn radiates energy, as infra-red radiation, back into space. A steady state is reached. This is a delicate balance that can be disturbed by changes to the system, in particular by changes to the quantities of various atmospheric gases. Methane is one example of a greenhouse gas. This traps some of the Earth's radiation, which would be re-radiated back into space. The effect is to make the Earth warmer. Other gases also let the Sun's visible radiation in but stop some of the Earth's infra-red radiation getting out. The two most significant greenhouse gases are carbon dioxide and water. CFCs are also very powerful greenhouse gases even in small quantities. There is a great deal of evidence that due to human-made emissions of greenhouse gases, the Earth is getting warmer, i.e. there is global warming.
Carbon dioxide and water are the combustion products of fossil fuels. Water does have a beneficial effect on global warming in that much of it present in the atmosphere is in liquid form, and is present in clouds which block out the Sun. At least half of the expected increase in greenhouse effect due to human activities is likely to be caused by carbon dioxide. Its proportion in the atmosphere is about 0.035% and is detected by infra-red spectroscopy. The proportion has been steadily increasing over the last 100 years. In this time the amount of fossil fuels burnt has increased by about 4% every year. The increase in concentration of carbon dioxide in the atmosphere should have been about twice as much as it actually has been. Carbon dioxide is taken up by the oceans and also by marine plants called phytoplankton.
When carbon dioxide dissolves in water, it forms hydrated CO2 molecules:
CO2(g) + aq CO2(aq)
A small proportion of the CO2(aq) goes on to react with the water:
CO2(aq) + H2O(l) HCO3-(aq) + H+(aq)
Since H+(aq) ions are formed, this reaction is responsible for the acidic nature of carbon dioxide.
It is thought that the concentration of carbon dioxide in the atmosphere will have doubled from the middle of the 19th century to the middle of the 21st century. As a result the average temperature will have increased by about 2oC. This will have a serious effect on the global climate. Sea levels have already risen due to ice melting, indeed, several of the Pacific Islands are already under water.
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